Re: Fw: missed itimer signals in 2.6

[Tom Marshall <tmarshall@real.com>]

[-- Attachment #1: Type: text/plain, Size: 1990 bytes --]

> >I understand what happens and why.  I admit that I'm not familiar with the
> >POSIX standard on this issue.  Questions:
> >
> > * I've heard that the kernel's timer resolution has increased from 10ms to
> >   1ms in 2.6.  Why does the itimer have such a large granularity?  I
> >   expected it to be highly accurate in this range.
> 
> I think it is.  The missing understanding is, I think, that you expect the 
> resolution to be exactly 1/HZ or 1ms.  It is actually not exactly that 
> because the PIC can not generate 1ms interrupts (close but not close enough 
> for NTP). So the kernel figures out what the true PIC rate is and sets up 
> the resolution for that.  This results in a resolution of ~999,849 
> nanoseconds (i.e. instead of 1,000,000 nano seconds per tick).  Now there 
> is some errors in converting this to micro seconds..., but the actual math 
> is done with more precision with the conversion after (which is why the 
> various times the program tries don't come out being exact multiples of 
> each other, or of anything expressed as only microseconds).

I expect there are at least a few applications that will misbehave because
the developers did not expect a timer to behave this way (regardless of
whether it's proper according to the spec).

Is it possible to choose a timer resolution that errs on the high side of
1ms instead of the low side? [*]  It seems to me that would result in the
application getting very close to the expected number of alarm signals.  I
am not at all familiar with the kernel design so I don't know if this would
be feasible or not.

[*] If this is the 8254 timer, using 1192 as a divisor should result in a
resolution of ~1,000,686 nanoseconds.

-- 
I mean, if 10 years from now, when you are doing something quick and dirty,
you suddenly visualize that I am looking over your shoulders and say to
yourself, "Dijkstra would not have liked this", well that would be enough
immortality for me.

[-- Attachment #2: Type: application/pgp-signature, Size: 189 bytes --]